Instrument Landing System (“ILS”) is a system supporting high precision guidance to an aircraft approaching and landing on a runway. The ILS typically includes transmitters and antenna arrays on the ground, receivers and antennas on the aircraft, and a display on the aircraft for the flight crew. Autopilots and/or flight directors may also be used on the aircraft.
The portion of the ILS that provides lateral guidance is referred to as the localizer. Vertical guidance is provided via a glideslope portion. The localizer and glideslope portions provide an aircraft with an indication of its separation from a desired approach path, in the form of an angular error referred to as an angular ILS deviation.
An aircraft preparing to perform a landing approach must fly a flight path that intersects the localizer. As the aircraft nears the desired approach path (i.e., the null of the localizer), it executes a turn to capture the null. This turn is typically executed in response to a steering input provided by a pilot following raw deviations on a display, in response to a steering input provided by a pilot following guidance received from a flight director system, or an autopilot system following guidance provided by the ILS. After a successful capture maneuver, the aircraft's flight path will be in line with the runway centerline. Ideally, the aircraft will perform a single turn to capture the localizer null, and will not fly through the null (overshoot) prior to completing its turn. The inherently angular nature of ILS deviations provides challenges when attempting to perform consistent localizer captures at varying distances from an ILS localizer transmitter.
On aircraft equipped with flight director and/or autopilot systems, converting the angular ILS deviation into a rectilinear ILS deviation is beneficial such that consistent localizer capture guidance can be provided regardless of distance from the ILS localizer transmitter. On some aircraft, the distance estimate that is used for converting angular ILS deviations to rectilinear ILS deviations is prone to error. The distance estimate is typically formed using radio altitude and glideslope error. Terrain effects, varying runway lengths, unusual glideslope angles, or localizer captures attempted prior to receiving valid radio altitude and/or glideslope deviations can result in an inaccurate distance estimate. This inaccurate distance estimate can, in turn, provide inaccurate localizer deviation and deviation rate data to the localizer control laws, and result in degraded localizer capture performance characterized by undesirable roll and/or yaw attitude profiles along with additional overshoot during the capture maneuver.
Moreover, the erroneous conversion factor used to convert an angular ILS deviation to a rectilinear ILS deviation manifests itself as a gain on the localizer deviation feedback loop in the control laws. This known source of inaccuracy requires control law gain reduction in the localizer deviation feedback loop, and the sacrifice of performance in favor of robustness. This known source of error also drives additional time and effort into the design and test of the control law, as the designer must show the control law is robust to a wide array of destination facility properties and approach geometries.
In practice, large localizer overshoots are common. This is primarily due to the fact that the segment of the localizer beam that reliably provides an accurate indication of aircraft displacement is relatively narrow. This segment, referred to as the course guidance sector, may be only approximately +/−2 degrees of arc about the localizer null. If an aircraft does not begin its turn until it encounters this sector, it may have a smaller physical distance than required in which to complete its turn in order to avoid an overshoot.
The propensity of an overshoot is exacerbated if the aircraft is intercepting the localizer with a large intercept angle, a high ground speed, or is close to the airfield (where the constant angular beam width corresponds to a smaller physical distance). Current aircraft systems do not begin the localizer capture maneuver until the linear part of the localizer beam is reached (i.e., the course guidance sector), and thus are prone to large overshoots.
It is with respect to these and other considerations that the disclosure made herein is presented.